This disclosure relates to imaging systems and, in particular, to imaging systems for transferring marking material through pattern-wise heating.
Printing technologies fall into two distinct groups: those that are digital and allow every printed page to contain variable text and images and those that are master plate based and allow high volume duplication of a single image. Common examples of digital printing technologies include inkjet, electrophotography (EP), and thermal transfer. Common examples of master based duplications technologies include offset lithography, flexography, and gravure.
Unfortunately, all of the digital printing technologies are severely limited in speed as compared to the master based duplication processes. This speed limitation reduces their productivity and fundamentally limits their economics to copy run lengths no larger than a few hundred copies. In the case of ink jet printing, inks consist of very dilute pigments or dyes in a solvent containing and print speed is limited by energy require for solvent evaporation. In the case of electro-photography, print speed is limited by the energy required for toner fusion. Finally, the print speed for thermal transfer is limited by the energy that is required to transform inked material on a ribbon from either a solid into a liquid or for the case of dye diffusion thermal transfer (D2T2), the energy from a solid to a gas. A large amount of energy is required for these thermal methods because the ink must be raised above a phase change temperature and the latent heat of melting or evaporation must be delivered. In addition to these considerations, the lower pigment concentration of typical digital marking materials leads to higher marking pile height. This is undesirable in terms of gloss uniformity, tactile feel, stacking thickness for books, and fold fastness. Furthermore, each of the digital marking materials usually has a much stricter limitation on color gamut and substrate latitude and size when compared with offset lithography.
Unlike the digital printing technologies mentioned above, lithographic offset printing uses very high viscosity inks in the range of 100,000 cp and above. In addition, these inks have high pigment loading with very little pile height. Very little energy is needed to fix these inks to paper such that very high production speeds can be achieved without excessively large drying ovens. In offset lithography a master plate is created which has hydrophilic and hydrophobic imaging regions. Such a plate is prepared off line and then mounted onto an imaging cylinder by wrapped it around the outside surface under tension. A fountain solution, often based on water, is first applied to this plate and selectively wets the hydrophilic regions. The imaging plate then comes in contact to a donor roller which provides a blanket layer of offset ink. The areas of the master plate wetted by the fountain solution reject the offset ink from the donor roller. These non-image regions are able to repel transfer of the offset inks due to hydrophobic nature of offset inks as well as the shear forces of the nip region which induce film splitting within the fountain solution. Once the master plate is selectively inked in the hydrophobic imaging regions, this inked image is then transferred to a rubbery offset cylinder which comes in contact with a printed substrate such as paper.
Another variation off lithography offset printing is waterless offset printing. In waterless offset technologies, the master is composed of a patterned polydimethylsiloxane (PDMS) layer, commonly referred to as silicone, used to block the transfer of ink. That is, silicone is used to prevent the transfer of the ink. Under the rapid shearing forces of the NIP, the viscoelastic cohesive forces within the ink can exceed the surface adhesion force at the silicone interface and the ink peels off from the non-image areas of the cylinder in a manor similar to a sticky yet elastic rubber like material. The adhesion force of the silicone interface is further reduced by the fact the silicone surface forms a “weak boundary layer” with solvents which diffuse into it and this promotes film splitting at the silicone interface. This behavior is amplified as the printing speed is increased because the shear forces act over a time scale faster than the inks can plastically deform. In non-silicone regions the adhesive forces overcome the built-in cohesive forces of the ink and the ink film splits apart thus leaving behind a layer of ink in the imaging areas.
In most conventional and waterless offset printing systems, the ink splitting between the donor and imaging plate and the imaging plate and the offset roller is approximately 50/50. In practical terms, this means that roughly 10 blank pages are need to remove enough ink from the offset cylinder so that the previous image is no longer visible. Thus these splitting dynamics lead to image ghosting when a new lithographic master plate mounted. Thus the ink splitting dynamics preclude lithographic technologies from achieving variable data short run printing jobs without significant image ghosting. However most offset printed jobs are long run and image ghosting does not significantly impact productivity as more make ready paper is needed to tune the alignment of each master plate corresponding to each color separated image.
Because of this issue and other issues with high viscosity inks, there have only been a few attempts at high quality high speed variable data digital printing with higher pigment concentration inks. Gravure and flexography inks with viscosities in the range of 50-1000 cp have been shown to respond to electrostatic pulling over short distances. However, the electrostatic forces are too weak to work with high viscosity high pigment concentration offset inks with viscosities above 100,000 cps.
Currently, no imaging technology exists that can print highly viscoelastic marking materials such as offset or waterless offset inks (i.e. marking materials having dynamic viscosities of 10,000-1,000,000 cps) in a digital fashion with variable data on each and every page.